Wireless heart rate monitors are commonly used during exercise, athletic competitions, medical tests and other activities. For example, a non-invasive heart rate monitor can be worn by a user, contacting the user at a suitable location such as the wrist. Invasive heart rate monitors, which are implanted in the body, can also be used. One example of a wrist-worn heart rate monitor transmits an ultrasonic signal toward the wrist and detects reflections of the signal from a blood vessel within the wrist. Based on the Doppler principle, a motion of blood in the vessel and/or the vessel itself can be detected and used to determine a heart rate of the subject. However, accuracy of the heart rate measurement can be impaired due to factors such as noise, muscle movement and low signal-to-noise ratio.